Astronomy & Science

Progress, Volume VI, Issue 12, 2 October 1911, Page 839

Astronomy & Science

Notes on Eclipses of the Sun By Joseph Brooks, P.R.A.S., P.R.G.S. When any one body obscures the view of another, the latter is said to be “eclipsed,” either partially or wholly, according as that view is partially or wholly obscured But, as a rule, the term “eclipsed” is confined to the interposition of one celestial body between two others. Thus, an “eclipse of the moon” happens when the Earth is in line between the Sun and the Moon, and “an eclipse of the Sun” happens when the moon gets ex-

actly in line between the Sun and the Earth. In astronomical importance a lunar eclipse cannot compare with a solar eclipse, and herein the lunar will be neglected. The Moon's orbit—or path through the heavens—does not lie in the same plane as the Earth's orbit, but it is inclined to it at an angle which may be as large as 5 : 20' or as small as 4° 58'. Thus, suppose the Moon, when "new," or just born, happened to be as far as possible "north" of the ecliptic, then it would gradually approach the ecliptic until in about a week's time it would cross it, and, continuing on, in another week it would be as far south as it could get, when it would, as it were, turn back, and in about a fortnight's time it would return to near its starting point. The two points where the Moon's path crosses the ecliptic are called the Moon's nodes. if the Moon and Earth followed the same path in the heavens, there would be an eclipse of the Sun every time there was a new moon, but owing to this movement of about five degrees either side of the

elliptic, the Moon will pass sometimes above and sometimes below the Sun. When, however, the sun happens to be near one of the nodes at the time when the Moon is crossing the ecliptic, then there will be an eclipse, and the nature of the eclipse will vary Avith the positions and apparent magnitudes of the Sun and Moon. Suppose that the centre of the Moon passed exactly over the centre of the sun —as seen by an observer in Wellington then, if the Moon's disc is "apparently" bigger than the Sun's, it is manifest that the Sun will be entirely hidden from

view. This would be a "Total eclipse of the Sun," and its duration would depend on how much bigger the Moon's disc apparently is. Should the two discs appear to be exactly the same size, there would be "total eclipse" but of only a moment's duration. If the Moon's disc was apparently "smaller" than the Sun's then there would be an "annular" eclipse, so called because there would be a "ring" of sunlight all round the moon. The Moon's distance from the Earth varies between 252,972 and 221, miles; manifestly the closer it is to the Earth the larger it will "appear" to be. The Earth's distance from the Sun varies between 91 and 94 millions of miles, and the Sun appears smaller the further it is away from the Earth. Now, when the Moon is at its nearest point and the sun at its farthest —if there is onethe Eclipse will be of longest possible duration to an observer on the Equator, and, under the most favourable conditions, the greatest duration will be 7 minutes 58 seconds. The shadow of the Moon on the surface of the Earth is only about 167 miles wide, so that an observer, say 90

miles to right or left of the path of the shadow would see a "partial" eclipse, whilst an observer, just inside the shadow's path would have a total eclipse of briefest duration. The Moon travels along its orbit about 2100 miles in an hour, and if the Earth did not rotate on its axis the shadow would pass the observer at that speed. But the Earth turns from west to east at the rate of 1040 miles per hour at the Equator, and where the shadow would pass the observer at the "difference" between these rates — is, at about 1060 miles in an hour. The further the observer is from the, Equator the quicker the shadow travels until, under certain circumstances, it would pass at something like 4000 to 5000 miles an hour. It has been calculated that in 100 years the Sun is totally eclipsed for about "twenty-four hours only," but taking into account the loss by clouds and bad weather, and the fact that a solar eclipse is only visible from a limited portion of the Earth which might be near the frozen poles or the shadow might pass (as did the last one)

almost wholly over an ocean, only about twelve hours of eclipse are available in a century to astronomers in which to make observations which are possible at no other time. It will readily be understood from this why astronomers spare neither time, trouble, nor money in their endeavour to get successful observations of a total solar eclipse. The last total solar eclipse took place this year on April 29th (Eastern date), and five different parties went to Neiafu (in Yavau) to observe it. The most northerly site was chosen by Mr. J. H. Worth in gton, who was assisted by Messrs. Johnson and Cruiekshank, of Hobart, and by Mr. J. Short, photographer, of the Sydney observatory. Mr. P. Barrachi, of Melbourne Observatory, and leader of the Australian Party, had his instruments fixed about 100 yards further south than Mr. Worthington, both of them being on vacant allotments in the town of Neiafu. Mr. Barrachi was assisted by Mr. W. A. Cooke, Govt. Astronomer of Western Australia, Mr. Dodwell. Govt. Astronomer of South Australia, Messrs. Byrnes and Merfield, of

Melbourne Observatory and by Messrs. beattie, Moors and Holloway, oi Sydney, together with two or three amateur assistants. _ > Dr. btelanik erected Ins instruments on a low bill about a couple of hundred yards south-east oi the R.C. Church. The two British expeditions— one under the direction of the Rev. Father Cortie, of Stonyhurst College, the other under the J?™ of . Dr. W. J. S. Lock of the Solar Physics Observatory, South Kensington— selected a site on the north-west side of a low hill and near its summit., about two miles south of Neiafu. Lather Cortie was assisted by Mr. W. Mclveon, photographer at Stonyhurst College, and by Rev. E. F. Pigott, of St. ikis College Riverview, near Sydney, N.S.W. Dr. Lockyer s party consisted of 1. K. MeClean (England), the writer,

and W. E. Raymond (Sydney) Mr H. Winkelmami (Auckland), and Mr. Anderson (England), who joined the party cn route. _ In addition, the British expeditions had the co-operation and very willing assist??^o° f <S e offi f rs ; , and men of the li.M.b. ' Encounter, beginning with Captain Colomb, who had charge of the telescope for watching the progress of the eclipse, and who had to call out the very instant the eclipse began. The "Encounter' took the English ' contingent to V avau, arriving there about 2nd April, whilst the remainder of the party travelled by the Union steamer "Atua," which reached Vavau on the 4th of April, Thence on every one was busy building concrete foundations for coelostats and siderostats, levelling ground for the erection of horizontal cameras and spectrographs, pier for transit instrument,

stand for chronograph, etc., etc., all to be used in one way or another during the (then) coming eclipse. The instrumental equipment of Dr. Lockyer's party was a bin. prismatic camera, 7% feet focus, with four objective prisms of 45" angle, which Dr. Lockyer attended to. Lieut. Clover first operated a "Cusp" projector for indieating how far off the beginning of the totality was, then he went to a small spectrograph with a Thorpe replica diffraction grating. A 12in. siderostat refleeted light to these instruments. Mr. McCleau's 21in. siderostat (by T. Cooke & Sons, of York) supplied light to a 10-foot concave grating spectrograph having a Gin. aperture Cooke photovisual object glass of 31% feet focal length. At the focus of this lens was a 2in. slit by Hilger, and the concave grat-

mg had 14.438 hues to the inch, having been ruled with a Roland engine in 1889. The films used were 24 inches (nearly) long and 3'/ t inches wide, and exposed in a specially made dark-slide which held the half-dozen films at once, the exposure of each separate film being controlled by a rack and pinion in such a way that when the rack reached a certain place a stop indicated that that film was completely ready for exposure. A Sin. doublet worked by the Rev. R, Pestall (of 11.M.5. "Encounter"), Mr. "Winklemann's telephoto camera of equivalent focus of 63 inches, and a very short focus, large apertured camera (which works at the unsual focal length of 1.6) were all fed, the two last by auxiliary mirrors, from the 21in. siderostat. The writer had charge of the 16in. coelostat (lent to the expedition by the Government Grant Committee (England),

and which supplied light directly to the 16ft. coronagraph having a 4in Cooke photo-visual object glass" and giving an image of l%in. diameter, and to the Bft. coronagraph, having an object glass of 4 %in., and forming o an image about %in in diameter. The 16-foot was worked by the writer Lieut Clutterbuck attending to the Bft. An auxiliary mirror (10in x 6i/,iu., with corners cut off) reflected light "from the coelostat to the 42-inch transparent grating spectrograph with a 4in. object glass, and in front of which was placed a transparent Thorpe grating of 17,000 lines to the inch, giving a visible spectrum of 10 inches. Lieut. Mortimer had charge of this instrument Another auxiliary mirror (oval on shaped refleeted light to Mr. Raymond's telephoto camera, the lens of which had an equivalent focal length of 9ft., and giving an

image of nearly an inch in diameter. A 48in. focus 6in. doublet was mounted as an equatorial telescope which Mr Anderson looked after. 'Everything was ready and in good o- o j n o- order by the 27th' April and only a%.lcar sky was needed to make the expeditions complete successes. The morning of the 29th April broke with scattered clouds all over but towards the critical time they cleared off or, rather, thinned out apparently north and south of the island, whilst over the island it seemed to be all clouds The "beginning" of totality was seen by us then the clouds closed" in again, and although just glimpsed at intervals the eclipsed sun remained obscured until just about the middle of totality, when it was distinctly seen but only faintly, and it remained more or less obscured until after the eclipse was over

Only when the sun is nearly obscured is there anything of more than ordinary interest to note, although before then the lights and shadows begin to look peculiar. About a quarter of an hour before totality begins the darkness becomes very evident, the birds cease to twitter, fowls go to roost, flowers close up, animals go to rest, the temperature falls, and the larger, stars and planets shine out and, during totality, a very peculiar gloom seems to have settled over the face of nature. The darkness is quite unlike the darkness of night, but rather as though a veil of neutral tint had covered everything. During totality the Moon's disc appears to be surrounded by fringe or crown of silvery light (called the corona) rising to a height of from one to three hundred thousand miles and out of which shoot tongues of pale, rosy-coloured flames of varying shape, and called protruberances, or prominences. Astronomers direct their attention to the study of these phenomena, both by use of the camera, to register the shape, and by use of the spectroscope, to analyse and determine the constituents. None of the instruments which were fed by auxiliary mirrors gave any results, the clouds were too thick, the light from the main reflecting instrument was only faint, and this faint light being again reflected by the auxiliary mirror became too feeble to impress an image on the photographic plate. Of the instruments fed by the coelostat (or siderostat) direct, Mr. MeClean's 311/>ft. spectograph did not produce a single result, whilst Dr. Loekyer's 4 prism camera only gave two faint pictures. The 16-foot coronograph produced one faint and one fair image, whilst the 8-foot gave a faint picture, in each case, of the end of the eclipse. Mi-. McKeon, of Father Cortie's party, got several faint pictures, but the details have not yet been made public.

Mr. Worthmgton and Dr. Stefanik seem to have been much more fortunate in their selection of a site than the British parties, the former getting pictures which both Mr McClean and Father Cortie describe as "really _ magnificent, and the latter (Dr. Stefamk) told the writer that his results were quite satisfactory. It is understood that the Australian party was fairly successful, although no official report is yet available.

Photographs. No . 1 is ft ictnre of the total golar eclipse of January, 1908, and was taken at Flint Tsland ■ vr 0 ' , n . , iN(X I '~ M an . eel, i >se ll JS necessary to know the local time very exactly, amongst +1 „„, rt „„ „„ +i n „+ /i , J ' in other reasons, so that the observers shall . , ' „„„ ■ t>w~ vr« o " ot be * akeD W surprise. Photo. No. 2 * ho ™ the instrument which determines time h >' indicating the passage of a star acioss the meiedian. aciow, the meiedian.

No. 3 shows the chronograph. On the drum is wrapped a sheet of paper and, by means of electricity, the indicated passage of the star across the meridian is recorded on that sheet of paper, at the same time that a chronometer is recording the time, second by second. No. 4 shows a camera which is pointed straight at the sun and driven by clock work so as to travel .just as fast as the

sun. thus keeping the Sun's image exactly in the same place on the photographic plate. No. 5 shows the 2lin. siderostat, the mirror being almost in front of Mr. McClean—the third figure from the left. At the time when the photograph was taken the mirror was reflecting a cocoanut palm leaf to Mr. Wiiikelmann's camera. No. 6 shows Father Cortie and his men adjusting a coelostat.